1. Field of the Invention
The invention relates generally to the field of electron emitting devices. More particularly, the invention relates to gated field emission devices.
2. Discussion of the Related Art
Field emission (FE) of electrons from nanostructured graphitic carbon-based materials including single-1-3 and multi-walled4,5 carbon nanotubes (CNT) and carbon nanofibers6,7 (CNF) has been an area of intense investigation in recent years. This body of research indicates that these materials have several advantages over other candidate materials for FE applications, namely very low turn-on fields, Eto, for initiation of electron emission and extraordinary environmental stability8. Most of the work in this field has focused on measurements of the FE properties of these materials deposited or grown onto a variety of flat substrates using a vast array of different deposition and measurement techniques. However, very little work has been presented on integrated gated cathode structures using these materials as FE elements. Xu and Brandes9 presented the first operating CNT-based gated cathode device in 1998 employing disordered mats of multi-walled CNTs (MWNT) grown within electrostatic gating structures by thermal chemical vapor deposition (CVD) Wang et al10 reported the operation of a similar device fabricated by a novel technique using a paste of CNT material and conductive epoxy deposited into microfabricated well structures. Lee et al11 also have recently reported on the operation of gated cathode structures similar to those demonstrated by Xu and Brandes with minor improvements in the structure fabrication process and increased control of the in situ MWNT growth step.
The disordered mat CNT material in the gated cathode structures referenced above is likely to contain numerous FE sites; there are multiple CNT tips in each cathode and there is evidence that that FE can occur from sites located along the walls11. While all of these devices possess operating characteristics desirable in any FE device (i.e., low Eto and high brightness) these fabrication processes offer no way to precisely control the factors of location, orientation, shape or density of the emission sites. These factors complicate the construction of FE devices that produce a well-focused electron beam as required in applications such as electron microscopy or electron beam lithography.
Recently, we reported12 a technique for fabricating gated cathode structures that uses a single in situ grown vertically aligned CNF (VACNF) as an FE element. This technique is disclosed and claimed in copending U.S. Ser. No. 09/810,531, filed Mar. 15, 2001 (ID No. 0842, S-92,869). This technique offers a way to produce gated cathode structures that takes full advantage of the FE properties inherent to nanostructured graphitic carbon materials, while providing a deterministic way to control the point of emission. A technique for producing these devices using conventional wafer-scale microfabrication techniques would greatly enhance this technology.
Another problem with this technology has been the need to use lithography equipment with sophisticated alignment capabilities. Equipment with these capabilities is expensive. The use of this equipment also requires time for the alignment function to be completed, thereby further adding to the overall cost of fabrication. What is also needed, therefore, is an approach that obviates the need for lithography equipment with sophisticated alignment capabilities.
Heretofore, the requirements of precisely controlling the location, orientation, shape and/or density of gated nanostructure field emission cathode material within a device structure without lithography equipment having sophisticated alignment capabilities when fabricating a gated emission site have not been met. What is needed is a solution that addresses (preferably all of) these requirements.